Solid state imaging devices can include an optoelectronic converter that transduces light energy received, through an optical lens, into electrical energy. The optoelectronic converter is typically arranged in an array of pixels. Each pixel features a discrete photosensor that converts a respective portion of the received light signal into an electrical signal. The electrical signals produced by each photosensor are processed by pixel and other circuitry to render a digital image representing the source from which the light energy was received.
Ideally, light received by each photosensor travels directly from the source being imaged, through a pixel surface facing the light stimulus, and strikes the photosensor. In reality, however, light entering the optoelectronic converter is scattered by reflection and refraction by pixel structures. Consequently, an individual photosensor can receive stray light, such as light that is intended for neighboring photosensors in the array. This stray light, referred to as optical “crosstalk,” reduces the quality and accuracy of the rendered image. The problems associated with optical crosstalk become increasingly more evident as imagers become smaller and array pixel densities increase.
Optical crosstalk is particularly problematic in color imagers, in which each pixel assumes a specialized light-detecting role. The photosensor in a typical pixel is sensitive to a wide spectrum of light energy. Consequently, an array of typical pixels provides a black-and-white imager. Color filters can be used to limit the wavelengths of the light that strikes the photosensors. In color imagers, color filter mosaic arrays (CFAs) are arranged in the light paths to the photosensors to impart color-sensitivity to the imager. In most cases a three-color red-green-blue (RGB) pattern is used, although other patterns exist: three-color complementary YeMaCy, or mixed primary/complementary colors, and four-color systems where the fourth color is white or a color with shifted spectral sensitivity. The CFAs are arranged in a pattern, with the Bayer pattern being the predominate arrangement used. The result is an imager capable of rendering color images in the visible light spectrum.
Ideally, each photosensor will receive only those wavelengths of light which the photosensor is intended to convert. In reality, however, optical crosstalk between the pixels allows light directed to the blue color filter, for example, to strike a red color pixel, causing the red color pixel to register more red light than is actually present in the image being viewed. A similar problem occurs with green light striking a blue pixel, red on green, etc. In addition, CFA imperfections will allow additional crosstalk in the form of some blue and green light entering red pixels, and red light entering blue and green pixels, for example. These various types of crosstalk reduce the accuracy of the images produced.
Another problem, particularly in CMOS imagers, is known commonly as “pixel noise.” Certain types of pixel noise are produced due to differing physical and electrical properties of the various components contained in adjoining layers and regions of the pixel device-structure. For example, mismatched material-interfaces can become areas that “trap” electrons or holes. A silicon dioxide/silicon interface, for example, can include such “trap-sites.” Interfaces that involve substances having a higher silicon density than the substrate create a higher likelihood of “trap-sites” along the boundaries, particularly as compared to the silicon/gate oxide interface of a transistor, for example. Trap-sites also can result from defects along silicon dioxide/silicon-interfaces between the layer or region boundaries, as well as dangling bonds or broken bonds along the silicon dioxide/silicon interface, which can trap electrons or holes.
The trap-sites typically are uncharged, but become energetic by trapped electrons or holes. Highly-energetic electrons or holes are called “hot-carriers.” Hot-carriers can get trapped in the available trap-sites, and contribute to the fixed charge of the device and change the threshold voltage and other electrical characteristics of the device. Current generation from trap-sites inside or near a photosensor contributes to dark current (i.e., electrical current present in the photosensor in the absence of light) in CMOS imagers due to constant charge leaking into the photosensor. Dark current is detrimental to the operation and performance of a photosensor. Accordingly, it is desirable to provide an isolation technique that prevents pixel noise in the form of current generation or current leakage, for example.
CMOS imagers of the type discussed above are generally known. CMOS imagers are discussed, for example, in Nixon et al., “256×256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol. 31(12), pp. 2046-2050 (1996); Mendis et al., “CMOS Active Pixel Image Sensors,” IEEE Transactions on Electron Devices, Vol. 41(3), pp. 452-453 (1994); U.S. Pat. Nos. 6,140,630; 6,376,868; 6,310,366; 6,326,652; 6,204,524 and 6,333,205, the entire disclosures of which are incorporated herein by reference.
There is a need to reduce optical crosstalk and pixel noise in solid state imagers. Particularly advantageous solutions would provide improved light filtering without additional costs or processing steps, and potentially would reduce the number and extent of steps or components used in the manufacturing process. Methods and structures that reduce optical crosstalk, and improve color filter capabilities, will improve imaging system sensitivity and accuracy.